json_decode()返回空，但它是有效的json

$a = '[{"function":"error_handler","class":"LP","type":"::","args":[256,"Call to undefined method LP_pdo::get_rorzxx() on D:\MARK\htdocs\lessphptest\application\controllers\users.php (23)","D:\MARK\htdocs\lessphp\LP.php",210,{"e":{"type":1,"message":"Call to undefined method LP_pdo::get_rorzxx()","file":"D:\MARK\htdocs\lessphptest\application\controllers\users.php","line":23}}]},{"file":"D:\MARK\htdocs\lessphp\LP.php","line":210,"function":"trigger_error","args":["Call to undefined method LP_pdo::get_rorzxx() on D:\MARK\htdocs\lessphptest\application\controllers\users.php (23)",256]},{"function":"shutdown","class":"LP","type":"::","args":[]}]';
$a = json_decode($a);
print_r($a);

echo json_last_error();
$a = '[{"function":"error_handler","class":"LP"